1. Field of the Invention
This invention relates to the liquefaction of coal and similar carbonaceous solids and is particularly concerned with operations in which heavy bottoms produced by liquefaction are upgraded by pyrolysis.
2. Description of the Prior Art
Processes for the liquefaction of coal and similar carbonaceous solids normally involve contacting of the feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure. This results in partial breakdown of the complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. These are recovered from the liquefaction effluent, leaving a heavy liquefaction bottoms product which normally boils in excess of about 1000.degree. F. and generally contains suspended solid residues. The liquefaction bottoms may constitute as much as about 50% by weight of the total liquefaction products.
A variety of different systems for upgrading liquefaction bottoms have been proposed in the past. Among the most attractive of these is pyrolysis of the bottoms to produce gases, additional hydrocarbon liquids, and coke, followed by steam gasification of the coke to form hydrogen for use in the liquefaction operation. Studies have shown that such an integrated process has many potential advantages over other processing systems, particularly if a catalyst is added to the coke to accelerate the gasification rate. One difficulty associated with such operations, however, is that the pyrolysis step results in the production of a heavy bottoms fraction which has a nominal boiling point in excess of about 1000.degree. F. and a pour point of about 300.degree. F. or higher and may contain up to about 3 weight percent of ash and other solids and in excess of about 1 weight percent sulfur. It has been suggested that this material be recycled to the liquefaction stage of the process but studies have shown that much of the recycled material passes through the liquefaction reactor with the bottoms and is returned to the pyrolysis unit. Here it is largely converted to coke and gas. At most only about 50% of the pyrolysis bottoms is converted to distillable liquid products and hence the efficiency of the operation suffers.